1. FIELD OF THE INVENTION
The invention relates to a drive system for a carriage, which is fitted with a scanning or recording device, in a reproduction appliance. In the reproduction appliance, the scanning or recording device scans an original or exposes a recording material periodically along scanning or recording lines. The carriage is moved forward with the aid of a drive apparatus along a straight line at right angles to the scanning or recording lines. The drive apparatus contains an electric motor whose rotation speed is determined by a frequency of drive pulses which are produced by dividing a master clock by an integer factor, and contains an apparatus for converting the rotary movement of the electric motor into a linear movement of the carriage.
One example of such a reproduction appliance is a so-called internal drum recorder or exposure unit for recording information on a recording material which is lying on the inside of a cylindrical trough. The recording is often made by a focused light beam that is aimed at the recording material from a rotating deflection device that is disposed on the imaginary axis of the cylindrical trough. While the deflection device is rotating quickly, it is moved in steps or continuously along the axis, so that the recording material is exposed along helical or circular lines, predominantly with raster-image motifs.
Another example of a reproduction appliance is an external drum scanner for optical scanning of an original which is disposed on the outside of a cylindrical drum, in order to digitize image information located on the original. In this example, the drum normally rotates, while an optoelectric scanning device is moved slowly parallel to the drum axis.
In reproduction appliances such as this, the carriage which is fitted with the scanning or recording device and is moved axially is driven, for example, by an axially running threaded spindle which is rotated by an electric motor, which is frequently a stepping motor. Other drives operate, for example, with a steel strip or a cable, or a linear motor is used. The frequency of the drive pulses for the stepping motor must be finely adjustable and must be kept very constant during the scanning or recording process since even very small position errors can adversely affect the recording or scanning quality.
Conventionally, the drive pulses are obtained from a high-frequency master clock which is divided in a divider by an integer factor which is chosen such that the speed of the resultant feed movement is as close as possible to a desired feed rate. In order to allow the frequency of the drive pulses to be adjusted finely, it is either necessary to use very high master clock frequencies in the Gigahertz band, which can be processed only by using logic circuits based on ECL technology, or synthesizers are required, with analog phase lock loop (PLL) chips which can divide both the integer and fractional parts.
These techniques require a relatively high level of complexity and, furthermore, are associated with problems. With Gigahertz technology, it is difficult to develop electromagnetically compatible circuits, and with synthesizers jitter and drift phenomena can easily occur, which must in turn be compensated for with a great deal of complexity in order to achieve the necessary frequency stability.
It is accordingly an object of the invention to provide a drive system for a scanning device or a recording device for a reproduction appliance which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which, using comparatively simple devices, can produce a feed movement whose speed can be adjusted finely and can be kept highly constant.
With the foregoing and other objects in view there is provided, in accordance with the invention, a drive system for a carriage fitted with an apparatus being a scanning apparatus or a recording apparatus. The carriage is disposed in a reproduction appliance, and the apparatus performs one of scanning an original and exposing a recording material periodically along one of scanning lines and recording lines. The drive system contains a drive apparatus for moving the carriage forward along a straight line at right angles to one of the scanning lines and the recording lines. The drive apparatus includes an electric motor having a rotation speed determined by a frequency of drive pulses produced by dividing a master clock by an integer factor; an apparatus for converting a rotary movement of the electric motor into a linear movement of the carriage; and a device by which a duration of an identical number of the drive pulses can be changed by at least one period of the master clock in each operating period of the apparatus.
For a drive system according to the invention, the object is achieved by a device with which the number of drive pulses can be lengthened or shortened by one or more periods of the master clock in each scanning or recording period.
The technique of lengthening individual pulses, which have been obtained by integer subdivision from a master clock, by one or more periods of the master clock in order to adjust the frequency of the pulses very much more finely than the frequency interval between integer fractions of the master clock is known per se as xe2x80x9cclock stealingxe2x80x9d or a xe2x80x9cbinary fraction dividerxe2x80x9d technique. The invention also covers the action on the periodic master clock being synchronized to the scanning or recording period. This measure prevents the creation of interference frequencies in the drive pulses, which can lead to beating with machine frequencies or with the raster frequency which may, in turn, lead to visible and thus disturbing strip or Moire patterns.
According to the invention, the mean speed of the feed movement of the carriage at right angles to the scanning or recording lines can be adjusted very finely, even if the frequency of the master clock is not as high as would be necessary without xe2x80x9cclock stealingxe2x80x9d. Specifically, in addition to the integer factor that is used for dividing the master clock, two further factors are available which can be varied in order to set the desired feed rate. These factors are the number of drive pulses in each scanning or recording period which are in each case lengthened or shortened by one or more periods of the master clock, and the number of periods of the master clock by which the respective drive pulses are lengthened or shortened in each scanning or recording period. Master clock frequencies of less than approximately 100 MHz are thus sufficient for practical applications. These are frequencies that can be produced and processed without any problems using simple digital techniques such as TTL technology.
The frequency of the master clock itself can always be kept constant for the invention since even the process of accelerating the electric motor at the start of a scanning or recording process can be controlled by suitably varying the factors which govern the feed rate. A constant-frequency master clock can be produced and kept constant considerably more easily than a variable frequency master clock, as has been required until now.
The synchronization of the lengthening or, alternatively, shortening of individual drive pulses with the scanning or recording period is achieved in that the feed distance from one scanning or recording line to the next is always the same. There are thus no density fluctuations in a scanned or recorded raster pattern, which can lead to visible strip or Moire patterns.
According to the basic solution of the invention, individual drive pulses in each scanning or recording period can either be lengthened or shortened. The first of these alternatives is preferable for practical implementation of the invention by commercially available electronic components. Specifically, in an embodiment such as this, individual drive pulses in each scanning or recording period are lengthened by one or more periods of the master clock by masking out the same number of periods of the master clock in each scanning or recording period, with the remaining periods being subdivided by the integer factor to form the drive pulses. In this case, the integer factor by which the master clock is divided is chosen such that the frequency of the drive pulses will be just above the target frequency without masking.
Alternatively, a circuit is also conceivable in which the drive pulses are shortened instead of being lengthened. In this case, the integer factor by which the master clock is divided is chosen such that the frequency of the drive pulses would be just below the target frequency, without shortening. In one preferred embodiment of the invention, the electric motor is a stepping motor. The use of a stepping motor has the advantage that its rotation angle is strictly proportional to the number of drive pulses. Furthermore, the stepping motor can be driven more or less directly using the drive pulses. A high-resolution operating mode for the stepping motor is preferable, with finely graduated intermediate currents, which allow particularly low-resonance running. In this operating mode, approximately sinusoidal phase currents are produced for the windings of the stepping motor, from the square-wave drive currents.
If a stepping motor is used as the electric motor, mechanical damping is also required. This is provided by a mass which is mounted such that it can rotate and whose moment of inertia is considerably greater than the moment of inertia of the other rotating parts of the drive apparatus, and which is rotationally coupled through a flexible coupling device to the other rotating parts of the drive apparatus. Since the action on the master clock is virtually the same in each scanning or recording period, the remaining interference can be sufficiently well damped by the flexibly coupled rotating mass, so that no resonances can appear. Furthermore, the frequency of the remaining interference is so far above the mechanical resonant frequencies of the reproduction appliance that it can be reliably stated that no mechanical oscillations will be excited.
The flexible coupling device is preferably a friction clutch, which is obtained in a simple manner by an annular friction lining which acts on a centrally mounted disk, which forms the mass which is mounted such that it can rotate. Thus, apart from its mechanically simple construction, a friction clutch like this has the advantage of a uniform braking torque irrespective of the rotation speed, so that the oscillation-damping effect of the disk is available throughout the entire rotation speed range, that is to say for any desired scanning or recording frequency.
In principle, apart from friction clutches, other types of coupling are feasible which allow relative movement between the disk and the rest of the system, for example hydrodynamic couplings, in which case the rotating mass can be formed by the flow medium itself, ferrofluid couplings or rubber couplings. However, with the fundamentally possible alternatives to a friction clutch, it may be difficult to achieve uniform oscillation damping which is largely independent of the rotation speed.
A regulated DC motor can be used as the electric motor, as an alternative to the stepping motor. In this case, a rotation angle sensor is also required, whose measurement pulses are supplied to a control circuit which ensures that each drive pulse produces a constant rotation angle of the DC motor. Specifically, a clock disk is located on the shaft of the DC motor as the rotation angle sensor, from which disk a sensor is used to derive a clock whose frequency is proportional to the actual rotation speed of the motor. The drive pulses produced according to the invention are at a frequency that is proportional to the nominal rotation speed of the motor. A phase comparator is used to compare the two frequencies, using normal control techniques, with a control variable being obtained from this, for readjustment of the motor. No mechanical damping is required for such a DC drive.
The invention is suitable, for example, for internal drum reproduction appliances, for example internal drum recorders or internal drum scanners, in which the recording material or the original is disposed cylindrically and is exposed or scanned line-by-line by a rapidly rotating light deflection device, with the light deflection device being moved slowly along the cylinder axis.
The invention is also suitable for other reproduction appliances, for example those in which, rather than the deflection device or a corresponding part of a scanning or recording device, this is done by rotating rapidly a drum on whose inside or outside the original or the recording material is located, with either the scanning or recording device or the drum being moved slowly forward axially by the electric motor.
Furthermore, the invention is suitable for all reproduction appliances in which a slow and a rapid relative movement take place between the scanning or recording device and the original or the recording material, with the rapid relative movement normally being at right angles to the slow relative movement. In all these apparatuses, the linking between the two axes according to the invention is feasible, namely the link between the scanning or recording line (rapid relative movement) and the feed direction (slow relative movement) in order to achieve the described advantages.
Therefore, the invention is not limited to internal or external drum recorders or exposure units, but is also suitable for those recorders or exposure units in which the original or the recording material is not disposed cylindrically, or is disposed cylindrically only in places. These include, for example, flat-bed or capstan exposure units or recorders. In exposure units such as these, a film to be exposed is stretched over a flat table, or is moved slowly over a drum. The light beam used for exposure is preferably deflected by a rapidly rotating polygonal mirror or by an oscillating mirror transversely with respect to the feed of the table or of the drum, and is imaged via an objective on the film. Capstan exposure units can be used to expose film strips of xe2x80x9cany desiredxe2x80x9d length.
In accordance with an added feature of the invention, the integer factor, a number of the drive pulses in the operating period, and a number of periods of the master clock by which a duration of the operating period is changed, are chosen such that a mean speed of a resultant feed movement is as close as possible to a desired feed rate.
In accordance with an additional feature of the invention, the master clock has a frequency of 100 MHz or less and the frequency of the master clock is constant.
In accordance with another feature of the invention, an identical number of the periods of the master clock are masked out in the operating period, with remaining ones of the periods of the master clock being subdivided by the integer factor to produce the drive pulses.
In accordance with a further feature of the invention, the electric motor is a stepping motor, and the drive apparatus has a flexible coupling device, a mass, and further rotating parts with a moment of inertia. The mass is mounted such that it can rotate and whose moment of inertia is considerably greater than the moment of inertia of the further rotating parts of the drive apparatus, and the mass is rotationally coupled through the flexible coupling device to the further rotating parts.
In accordance with another added feature of the invention, the flexible coupling device is a friction clutch and the friction clutch contains an annular friction lining acting on the mass. In addition, the friction clutch has a universal-joint attachment for pressing the friction lining.
In accordance with another additional feature of the invention, the electric motor is a DC motor. A rotation-angle sensor is provided for sensing a rotation angle of the DC motor, and a control circuit for controlling a rotation speed of the DC motor is provided.
In accordance with a concomitant feature of the invention, the reproduction appliance is an internal drum, an external drum, a flat bed or a capstan type.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a drive system for a scanning device and a recording device for a reproduction appliance, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.